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- Discussion:
    - haversian systems are of two types, primary and secondary;
    - primary systems are formed on the growing surface of bone; periosteal vessels running in long axis of bone become surrounded by bone, 
         first in a trough, then completely buried in canal;
    - this canal is at first much larger than vessels;
    - osteoblasts line canal, &, by producing new concentric rings of bone at its periphery, gradually narrow the lumen until vessels, now in a 
         much smaller canal, come to lie within the concentric lamellae of bone that characterize an Haversian system;
    - it should be noted that these systems are formed from periphery of canal toward the center;
    - secondary haversian systems are formed in newly eroded channels within depths of the cortex;
    - constant internal remodeling of cortex of bone takes place throughout life;
    - osteoclasts from surface or from adjacent canals resorb bone to form new channels within the cortex;
    - these channels are referred to as resorption canals;
    - advancing front of the resorption canal is called the cutting cone;
    - vessels follow osteoclasts into these resorption canals; later, canal walls become lined w/ osteoblasts, & new haversian system is formed;
    - this continuous alteration of internal architecture of bone enables it to adapt to changes in stress & metabolic demands throughout life